The present invention relates to a voltage controlled oscillator with improved tuning linearity.
A voltage controlled oscillator VCO is an oscillating circuit whose output frequency changes in accordance with a control voltage supplied to the circuit. The VCO is typically used as one of the components in a phase locked loop PLL. A phase locked loop is often used when both the phases and the frequencies of two signals must be synchronized. Both the phase and the frequency of the VCO output signal are locked to a reference signal or a control voltage signal, whereby the output signal will follow the changes of the reference signal. The output signal, or a signal obtained by dividing it, is compared to the reference signal in the phase comparator of the loop. The phase difference of the signals is transformed using a phase comparator circuit and a filter into an error voltage which is supplied as a control voltage to the voltage controlled oscillator.
The control voltage either increases or decreases the frequency of the oscillator so that the loop is locked. In the locked mode the phase comparator will not output any pulses, so that the control voltage generated by the loop filter remains at a constant value. The slope of the curve describing the VCO's frequency dependence on the control voltage is called the VCO factor. As a general rule we could say that the higher the VCO factor the greater are the noise problems in the loop, and it is difficult to obtain a clean frequency.
A voltage controlled oscillator can be realized in many different ways. Usually it comprises a transistor coupled as an oscillator. An internal capacitance of the transistor may function as the feedback maintaining the transistor's oscillation, whereby the oscillation frequency may range from 1 GHz to 10 GHz using a bipolar transistor, and even higher frequencies are obtained using a FET. A series resonance circuit is arranged at one of the oscillator transistor electrodes, usually at the base (gate) so that it is possible to control the oscillation and to have the circuit operating as a voltage controlled oscillator VCO. According to the desired characteristics this could be designed in different ways, but basically it comprises a capacitance diode, an inductance, e.g. a strip line with one end usually shorted, and the internal capacitance of the transistor. The capacitance diode is generally coupled in series with the strip line, or in parallel with it, and then the anode of the diode is earthed (i.e., grounded) regarding the direct-current voltage. When the control voltage provided by the PLL loop filter (or any other external control voltage) is supplied to act on the cathode of the capacitance diode then the voltage over the diode changes and thus also the capacitance of the diode. Correspondingly the capacitance of the whole oscillator circuit will change, and therefore the amplified oscillating signal frequency provided by the transistor will change in accordance with the control voltage. Without any further linearization measures the linear control range of the VCO is limited when a varactor (capacitance diode) included in the resonance circuit has a non-linear voltage/capacitance characteristic. The modulation sensitivity or the tuning sensitivity of a voltage controlled oscillator, such as a Colpitts oscillator, will change when the tuning voltage changes, and usually its highest non-linearity is at voltages below 2,5 V. Generally this is compensated for by suitable capacitors or capacitance diodes connected in series. The arrangements to cancel the non-linearity of the varactor increase the linear control range of the VCO. Certainly an arrangement of this kind often will limit the total control range. It is also possible to use e.g. a negative bias voltage to the varactor in order to improve the linearity. However, this method will operate properly only with low supply voltages and on a high tuning frequency band.